A typical process for the aqueous dispersion polymerization of fluorinated monomers includes feeding fluorinated monomer to a heated reactor containing a fluorosurfactant and deionized water. Paraffin wax is employed in the reactor as a stabilizer for some polymerizations, e.g., polytetrafluoroethylene (PTFE) homopolymers. A free-radical initiator solution is employed and, as the polymerization proceeds, additional fluorinated monomer is added to maintain the pressure. A chain transfer agent is employed in the polymerization of some polymers, e.g., melt-processible TFE copolymers to control melt viscosity. After several hours, the feeds are stopped, the reactor is vented and purged with nitrogen, and the raw dispersion in the vessel is transferred to a cooling or holding vessel.
For use in fluoropolymer coating applications, polymer dispersion is typically transferred to a dispersion concentration operation which produces stabilized dispersions used as coatings for metals, glass and fabric. Certain grades of PTFE dispersion are made for the production of fine powder. For this use, the dispersion is coagulated, the aqueous medium is removed and the PTFE is dried to produce fine powder. Melt-processible fluoropolymers for molding resin use are also coagulated and dried and then processed into a convenient form such as flake, chip or pellet for use in subsequent melt-processing operations.
As described in U.S. Pat. No. 3,391,099 to Punderson, dispersion polymerization involves two generally distinct periods or phases. The initial period of the reaction is a nucleation phase in which a given number of polymerization sites or nuclei are established. Subsequently, there occurs a growth phase in which polymerization of fluorinated monomer on established particles occurs with little or no formation of new particles. Successful production of the high solids fluoropolymer dispersion generally requires the presence of the fluorosurfactant, especially in the later growth phase of polymerization in order to stabilize the dispersion preventing coagulation of the fluoropolymer particles.
Fluorosurfactants used in the polymerization are usually anionic, non-telogenic, soluble in water and stable to reaction conditions. The most widely used fluorosurfactants are perfluoroalkane carboxylic acids and salts as disclosed in U.S. Pat. No. 2,559,752 to Berry. Because of recent environmental concerns with regard to perfluoroalkane carboxylic acids and salts, there is interest in reducing or eliminating perfluoroalkane carboxylic acid surfactants in fluoropolymer polymerization processes.
Perfluoroether carboxylic acids and salts are disclosed in U.S. Pat. No. 3,271,341 to Garrison and U.S. Pat. No. 3,391,099 to Punderson for use in the aqueous polymerization of fluorinated monomers. U.S. Pat. No. 4,864,006 to Gianetti et al. discloses the polymerization of fluorinated monomers in the presence of a perfluoropolyether having neutral end groups, a perfluoropolyether oil, in the form of an aqueous microemulsion. The aqueous microemulsion can be prepared using a perfluoropolyether having carboxylic end groups or cationic end groups. In U.S. Pat. No. 6,395,848 to Morgan et al., aqueous dispersion polymerization of fluorinated monomers is improved by using a combination of fluorosurfactants, one of which is a perfluoropolyether carboxylic acid or sulfonic acid or salt of either.
Wille et al. in U.S. Pat. No. 6,841,616 have proposed the use of siloxane surfactant in the aqueous polymerization of fluorinated monomers in an attempt to reduce or eliminate perfluoroalkane carboxylic acid surfactants. However, Wille et al. demonstrate the invention in the Examples only for homopolymers and copolymers of vinylidene fluoride.